Update: makes sense now after seeing UbonJoe's explanation above.
I would hope you could get a new "application under consideration" with a new report back date. I would be nervous without that. One would hope that if your passport is viewed by any police authority they would be able to contact your immigration office and find out that you are still under consideration, but it would make for a very unpleasant wait for it to get straightened out. Immigration should understand that and be amenable to giving you a new under consideration date.

Yes, you need to have a Bangkok Bank account somewhere in Thailand, you don't actually need (nor can you get an account at the Bangkok Bank NY Branch). Money from the US sent to Bangkok Bank NY Branch via the ACH system (using IAT format) will be forwarded from NYC to Thailand where it will be deposited into your local Bangkok Bank branch account. You can sign up for free SMS notification of when the international transfer is received into your Bangkok Bank local branch account.
There's a lot of information at the link that was posted but you have to expand all under Service Details in order to see it.

Google: car rent phuket site:forum.thaivisa.com
I prefer Google search over the internal search capability on thaivisa. "rent" gets "rental" as well. The site:forum.thaivisa.com gets results only from the thaivisa site.
Pay attention to the dates. You will get a lot of information that is probably out-of-date.
Among the results:
Happy searching.

There are circumstances under which earned interest is withheld, but if that represents the only income you have in Thailand it is often possible to be refunded the entire amount withheld if you, in fact, don't have a tax liability.
When you earn more than 20,000 baht interest in a year or you are paid interest for a fixed deposit, 15% of such interest will be withheld automatically. To get this interest back you can annually obtain tax withholding documents from your bank, obtain a Thai Tax ID (which only needs to be done one time) and file a Thai Personal Income Tax Return.
Assuming you don't have any other income you will receive back the entire amount of interest withheld from Thai Revenue Department after your return has been processed (takes on the order of three to six weeks). The forms are in Thai, but often you can get assistance in English at your local Thai Revenue Department office. I was personally surprised at how helpful my local office was. There's a big learning curve the first year, but thereafter it amounts to doing the same things you did the last year.
It's possible to file online, but again the form is in Thai and you would probably need to be able to read Thai and/or have a Thai-speaker help with your return. There is a app available (RD Smart Tax) for your smart phone that might be in English that you could use to file your tax return, but I don't have any experience with that. It's my understanding that some tax offices will actually accept a simplified form for reclaiming your withheld interest, but I have always had to file a full Personal Income Tax Return - P.N.D. 90 (ภ.ง.ด. 90).
The above is not rocket science but it is work. So if you would like to avoid the work and take an easy way out I would recommend the Mee Tae Dai (MTD) savings account at Krungsri Bank which is currently paying 1.3% interest. Significantly this account is not a fixed deposit and therefore doesn't trigger the automatic withholding of interest. Understand that interest will be withheld at 15% if at any point in the year you exceed 20,000 in earned interest (it's not clear to me if it is 20,000 interest earned anywhere in Thailand, 20,000 earned at the same bank, or 20,000 earned from the same account, but in any case if you earn more than 20,000 interest you can expect to have the 15% withholding kick in). You would need to keep about 1.5 million baht in the MTD account (at 1.3% interest) to exceed 20,000 interest which would trigger the 15% withholding, so merely having more than 800,000 in the account will not automatically trigger the withholding.
Obviously, you can find fixed deposit accounts that are paying more than 1.3% interest but bear in mind that using a fixed deposit account will trigger automatic withholding of interest (in that account) from the first baht earned.

I have always understood the requirements for lump-sum deposit for the purpose of extension of stay to be two-fold: 1) it must be immediately available to be withdrawn with only a possible loss of interest as a penalty, and 2) the principal must not be subject to market fluctuations.
Even if the selling of mutual fund shares was a streamlined experience and occurred relatively quickly it wouldn't matter as they fail the second requirement, that is, they are by definition subject to market fluctuation. It is possible to buy mutual fund shares at one price and sell them at another price and be subject to market gains or losses. Savings accounts and fixed deposits are not subject to such market fluctuation of the principal.
Now that the new "don't touch" rule is being applied to 400,000 of the 800,000 lump-sum deposit for the entire year, maybe the immediately available rule will be relaxed, but I'm not holding my breath.

Phuket Immigration has always required of me for my retirement extension both a bank letter and a stamped bank-produced statement. I would think that would be sufficient because the bank statement is more accurate (due to possible consolidated entries) than a passbook. You might want to check with Phuket Immigration but I would say it's worth the risk. Worst case is you will have a bank statement and the IO will want to see your passbook, then you can explain this account doesn't have a passbook and the IO can call your bank to confirm that fact.

I don't disagree with your points which are all valid.
The Mee Tae Dai account was better suited to using the money in the account during the non-seasoning months of the year (3 months prior to application for extension of stay) than a fixed deposit. Due to the changes in policy this year that advantage is greatly reduced, but we can still use the amount in excess of 400,000 during half the year.
The simplicity, convenience, and ease of the MTD account is purchased at the cost of a lower interest rate.
Which is a better deal depends on individual circumstances and what factors are more important to a person. Undoubtedly you can achieve higher interest rates in fixed deposit accounts.

The Mee Tae Dai savings account at Krungsri Bank offers an almost fixed deposit-like interest rate (currently paying 1.3%) in a regular savings account (with a passbook). Advantages of this account: no automatic 15% interest withholding for taxes; consequently, no need to file a Thai Personal Income Tax Return to claim back the interest withheld; interest calculated daily and paid monthly (unlike fixed deposits you won't lose any interest you have earned if you have to make a withdrawal); account doesn't expire so you don't have to roll into anything, it just sits there, no fuss, no muss.

Bangkok Bank will stop accepting ACH transfers April 1, 2019, pursuant to a crack down by the US Government on ACH transfers that result in money being transferred outside the US. As of this time no one can identify any US financial institution that will allow retail customers to send ACH transfers using the new international IAT format. Some pension providers and other commercial businesses do use the IAT format already, but this facility has not been made available for retail customers.

The issue is that the US Government has cracked down on ACH transfers that result in money going outside of the US which is what Bangkok Bank NY Branch is doing (you send a domestic ACH and it ends up in Thailand). Consequently as of April 1, 2019 Bangkok Bank NY Branch will no longer be able to do this unless the ACH is sent in a new international format (IAT, which contains more data about where the money is going). Some pension providers and other commercial businesses already send ACH transfers in the new format (IAT), but no one has to my knowledge identified any financial institution (presumably including your US bank) that allows retail customers like yourself to send an ACH using the new format (IAT). Post April 1, you will no longer be able to send money via ACH to Bangkok Bank's NY Branch and have it forwarded to your bank in Thailand.
You can continue to wire money to your bank in Thailand using SWIFT (although the costs may be greater than the old ACH method). There are ways to check whether your pension is already being sent to Bangkok Bank using the new IAT format.

Yes, I agree it does seem to matter how you access TurboTax. From the Lookup Tool at the IRS Free file site seems to produce the best results in terms of getting it free, although I believe I tried that last year and was unsuccessful. It did work this year and I will try the same thing next year.
I think some are misled into going to the TurboTax site directly and while their return may start out being free at some point the complexity of their taxes (sometimes something as simple as having capital gains) will drive them into a more expensive option. Hopefully, this won't happen if you go through the IRS site. I think it's worthwhile to go through the extra step at the IRS site on the chance that it might be free that way.
They do ask for a Driver's License or similar, but are currently accepting "have no IDs." I hope they continue to be relaxed about that. Time will tell.